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I am still having some difficulty in understanding what Mike Daly is telling
us..
I had pointed to predictions of the Moon's position, and particularly to the
"Connaissance du Temps", available in Halley's time, and asked-
| > Are those what Daly appears now to dismiss as "raw data"?
And he has replied-
| Of course they are raw data. Halley did not go into his friendly,
| neighbourhood navigator's supply shop and buy a printed copy of the
| lunar information telling him when a particular star would be occulted
| _in_the_future_ by the moon. He had data that specified the position
| information on the moon - in the past. That's raw data. He has to use
| it along with a model of lunar motion to produce the information he needs.
Well, that's exactly what he could do, back in 1698. He could buy a copy of
Connaissance du Temps, for 1699, at Stationers' Hall, in London, which would
provide him with the position of the Moon in the sky, in ecliptic latitude
and longitude. I've little doubt that he did. I don't have a copy available
to tell me whether it would supply occultations or not (they are rather
local phenomena, which would have applied to Paris, not to mid-Atlantic, in
any case). But Halley required close appulses, not occultations, as I have
explained before. He was perfectly capable of deducing those from
predictions of the Moon coordinates and his own detailed knowledge of star
positions around the ecliptic. He had been doing that, routinely, for years.
But Mike Daly seems to be insisting that only information on past positions
of the Moon was available in Halley's time, and not predictions into the
future (I have tried to discover some alternative meaning, without success,
in his words quoted above, in case I'm accused once again of "misquoting or
convoluting" his meaning). If that is what he is claiming, it's nonsense.
These, in "Connaissance" and elsewhere, were predictions, not history. It's
perfectly true, as I've said before, that those lunar predictions in 1699
were not up to the precision of Mayer's, in Maskelyne's 1767 almanac. They
wouldn't have qualified for the longitude prize. There's little doubt that
Halley, from his own observations, could do better. But Daly is telling us
(isn't he?) that no such predictions existed.
If, as it seems, he is describing those predictions in "Connaissance" as
"raw data", that's simply abuse of language. Astronomer's real "raw data" of
Moon observations had been collected for many years, sifted and assessed,
fitted to the best model they could think of, and then that model was run to
produce future predictions for coming years. At least, that's how we would
describe their process in modern terms. "Raw data"?
Halley's proposal was to bypass much of that process by examining data from
one Saros cycle previous, if that detailed data had been available. It had
some strengths, and some weaknesses, as I have discussed. It was superseded
by better mathematicians than was Halley; Clairaut and then Mayer, who had
Newton's understanding of the dynamics to build on.
George.
contact George Huxtable at george---.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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